Previously, an electric power steering apparatus, which assists a steering operation of a driver of a vehicle, has been known.
For example, JP5462374B2 (corresponding to US2013/0140964A1) discloses an electric motor that serves as a drive device used in an electric power steering apparatus. In this motor, a rotatable shaft is fixed to a rotor, and an end part of the rotatable shaft, which is located on an output end side, is rotatably supported by a bearing, and an opposite end part of the rotatable shaft, which is opposite from the output end side, is rotatably supported by another bearing. Among these bearings, the bearing, which supports the opposite end part of the rotatable shaft, which is opposite from the output end side, is received in a bearing box installed to a case that receives the stator and the rotor. An O-ring, which is installed between the inner wall of the bearing box, and the bearing, absorb a vibrating force, which is exerted in a radial direction of the rotatable shaft due to a positional deviation between a center of gravity of the magnet installed to an outer peripheral surface of the rotor and an axis of the rotatable shaft, or unbalance between a magnetic force of the stator and a magnetic force of the rotor. In this way, the motor limits the generation of the vibrations and noises at the time of rotating the rotatable shaft.
However, in JP5462374B2 (corresponding to US2013/0140964A1), the O-ring, which is installed to the bearing of the motor, applies a load to the bearing box and the bearing in such a manner that the load is applied to the bearing box and the bearing in a uniform manner along an entire circumferential extent of the O-ring. Therefore, in order to withstand the vibrations generated at the time of transmitting the torque from the worm connected to the output end of the shaft of the motor to the worm wheel, it is necessary to increase the hardness of the material of the O-ring or to increase the amount of compression of the O-ring.
However, when such a modification is made, an installation load, which is exerted at the time of installing the bearing and the O-ring to the bearing box, is increased. This might possibly cause disadvantages, such as removal of the O-ring, cutting (severing) of the O-ring, or generation of an impression on the bearing. When such a disadvantage occurs, the vibrations and noises generated from the motor may possibly be increased.